A theoretical investigation of inversion layer transducers (ILT) for ultrasonic skin thickness measurement
The human skin is the outermost organ of the body. The thickness of the skin can give useful clinical information on its condition. In this thesis the complicated structure of the skin was described and methods of skin thickness measurement critically reviewed. Ultrasound was identified as a non-invasive method having several advantages over the other methods, such as safety, ease of use and low cost. The existing ultrasonic systems have not been successful to quantify the skin structure properly, due to lack of high resolution transducers and the biology of the skin structure. The PZFlex Finite Element Method (FEM) package was used to model the skin, an ultrasonic thickness measurement system, and their interaction, in order to define the system structure and parameters. Initially, the skin was modelled as one planar layer of a linear isotropic material and 5MHz transducer was used to minimize computational effort. A nonplanar structure was then modelled and compared to the planar interface to see the effect of the additional complexity on the backscattered signal and to clarify the axial and lateral resolution requirements for skin thickness measurement. Different front face configurations of the transducer were also modelled to investigate the effect of the geometry on the backscattered signal as a starting point to introduce practical coupling. A more realistic skin structure was then modelled at 80 MHz, by super-imposing a FEM mesh on a micrograph of the skin and the backscattered ultrasound signal from the real skin interfaces compared to a signal from planar interfaces. An appraise of the existing technology was concluded and the requirements for an ideal skin thickness measurement system were then addressed. A monolithic LiNb0b3s transducer incorporating inversion layers (IL) was modelled using PZFlex to investigate the usefulness of these transducers for skin measurement. A novel mathematical 1-D linear systems model was developed for a transducer incorporating one front face inversion layer. This model gave a physical insight into, and more understanding of, the transducer behaviour. It was found that, although ILT's offer improved sensitivity and bandwidth for skin thickness measurement, they suffer from similar problems to conventional devices and a new transducer technology is needed for ultra high resolution applications on real skin surfaces. The new theory offers substantial base for the design of ILT' s, which will have significant applications in other areas, such as harmonic imaging and multi-frequency sonar.